The present invention concerns improvements in catalytic materials, and more especially it concerns improvements in catalyst components which are capable of storing oxygen.
The use of automobile exhaust gas catalysts has contributed to a significant improvement in air quality. The most commonly used catalyst is the xe2x80x9cthree-way catalystxe2x80x9d (TWC) which has three main duties, duties, namely the oxidation of CO, the oxidation of unburnt hydrocarbons (HC""s) and the reduction of NOx to N2. Such catalysts require careful engine management techniques to ensure that the engine operates at or close to stoichiometric conditions (air/fuel ratio, xcex=1). For technical reasons, however, it is necessary for engines to operate on either side of xcex=1 at various stages during an operating cycle. When the engine is running rich, for example during acceleration, the overall exhaust gas composition is reducing in nature, and it is more difficult to carry out oxidation reactions on the catalyst surface. For this reason, TWC""s have been developed to incorporate a component which stores oxygen during leaner periods of the operating cycle, and releases oxygen during richer periods of the operating cycle, thus extending the effective operating envelope. This component is believed to be ceria-based in the vast majority of current commercial TWC""s. Ceria, however, especially when doped with precious metal catalysts such as Pd, shows a great tendency to lose surface area when exposed to high temperatures, eg 800xc2x0 C. or above, and the overall performance of the catalyst is degraded Because of this, TWC""s are being proposed and introduced in some demanding markets which use, instead of ceria as the oxygen storage component, ceria-zirconia mixed oxides, which are very much more stable to loss of surface area than ceria alone. Ceria itself is a rare earth metal with restricted suppliers and ceria-zirconia is a relatively expensive material when available commercially, and it would be desirable to find a material having at least as good oxygen storage performance as ceria-zirconia, but utilising less expensive materials.
Herein, reference will be made to various mixed oxides. It is now accepted in the art that for ceria-zirconia, the best oxygen storage performance is exhibited by xe2x80x9ctruexe2x80x9d mixed oxides, that is single phase mixed oxides, compared to mixed oxides which contain two or more phases.
The prior published U.S. Pat. No. 4,299,734 for instance describes the use of a stable catalyst composition which comprises platinum or palladium and a porous support consisting of zirconia and at least one oxide selected from cerium oxide, managese oxide and iron oxide. However, the oxides other than zirconium are only present in very small amounts ie 1-80 g/liter of zirconium oxide and hence have no oxygen storage capacity.
Similarly, WO-A-96 20787 discloses a catalyst for treating a gas stream containing (halogenated) organic compounds carbon monoxide and mixtures thereof, the catalyst comprising at least one platinum group metal, zirconium oxide and at least one oxide selected from manganese oxide, cerium oxide and cobalt oxide, the catalyst being substantially free of vanadium. However, this document also contains an excess of zirconium oxide in relation to the other oxides and the catalyst is used for treating gas streams rich in oxidising gases xe2x80x94unlike the reducing gases which are emitted by exhaust systems of automotive engines. Again, there is no mention of the catalyst composition having any oxygen storage capacity.